1. Field of the Invention
The present invention relates to an exposure device having a lens by which an image of an original is formed on a photosensitive material and of which an optical axis is disposed at a position shifted from the center of the original to an outer peripheral side of the original and having an illuminating means for illuminating the original.
2. Description of the Related Art
Conventionally, there has been known an image forming device in which an original is illuminated by a light source and an image of the illuminated original is formed on a photosensitive material by using an imaging lens.
On the other hand, the imaging lens has the characteristic referred to as COS.sup.4 Law. When the surface of a plane original which is perpendicular to an optical axis of the imaging lens is a uniform diffusing surface, the difference between an illuminance at a position in proximity to the optical axis on an image surface facing the plane original and an illuminance at a point separated from the optical axis on the image surface becomes larger and a problem arises in that shading occurs.
In order to correct the shading, there has been proposed an image forming device of a slit exposure type disclosed in Japanese Patent Application Laid-Open (JP-A) No. 58-68062. This image forming device is, as shown in FIG. 11A, provided with an original stage 1 which can move in a direction indicated by arrow E, a light source 2 for illuminating an original, a slit 3 which causes the light reflected from the original to becomes a slit light extending in a direction perpendicular to the direction in which the original stage 1 moves, a lens 4 which can move in the direction of the optical axis and by which an image of the original is formed on a photosensitive material, and a correcting plate 5 disposed on the side where light is emitted from the lens 4 and provided to be movable integrally with the lens 4, and as shown in FIG. 11B, a distribution of a quantity of light on an image surface in a direction corresponding to a direction along a longer side of the slit is made uniform by the correcting plate 5.
Further, there has also been proposed an image forming device of a surface exposure type disclosed in Japanese Patent Application Laid-Open (JP-A) No. 61-26036. In this image forming device, as shown in FIG. 12, a flash lamp 8 and a reflector 9 are disposed below each of corner portions 7A of a platen glass 7 fitted into an upper surface 6A of a housing 6 having a rectangular box-shaped configuration, and each of the flash lamps 8 illuminates an area which is about a quarter of a total area of the platen glass 7 including a corresponding corner portion 7A.
However, although the former image forming device disclosed in JP-A No. 58-68062 allows the distribution of a quantity of light on the image surface in the direction corresponding to the longitudinal direction of the slit to become uniform, the distribution of the quantity of light in the widthwise direction of the slit cannot be made uniform.
Further, in the latter image forming device disclosed in JP-A No. 61-26036, each flash lamp 8 and each reflector 9 must be formed and disposed such that the light of the each of flash lamps 8 illuminates an area which is about a quarter of the total area of the platen glass 7 including the corresponding corner portion 7A. For this reason, an illuminating system becomes complicated and the cost thereof increases.